[issue35866] concurrent.futures deadlock

[cagney]

cagney <andrew.cagney at gmail.com> added the comment:

@hroncok see comment msg339370

Vanilla 3.7.0 (re-confirmed) didn't contain the change, nor did 3.6.8 (ok, that isn't vanilla) but both can hang using the test.  It can take a while and, subjectively, it seems to depend on machine load.  I've even struggled to get 3.7.3 to fail without load.

Presumably there's a race and grinding the test machine into the ground increases the odds of it happening.

The patch for bpo-6721 could be causing many things, but two to mind:

- turning this bug into bpo-36533 (aka bpo-6721 caused a regression)
- slowed down the fork (sending time acquiring locks) which increased the odds of this hang

My hunch is the latter as the stack dumps look nothing like those I analyzed for bpo-36533 (see messages msg339454 and msg339458).

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